INFLUENCE OF CERTAIN FACTORS ON THE AMMONIUM AMALGAM EFFECT IN THE APN METHOD

A. V. KONKOVA, A. G. STROMBERG

(Presented by the scientific-methodological seminar of the KhTF)

In works [1, 2, 3] it has been shown that the sensitivity of the method of amalgam polarography with accumulation (APN) can be increased by more than an order of magnitude through the use of the ammonium amalgam effect (AAE). The ammonium amalgam effect consists in the fact that, during electrolysis of an ammonium salt solution at a sufficiently negative potential on a mercury cathode, an unstable ammonium amalgam is formed which, upon decomposing, gives a gaseous mixture of ammonia and hydrogen. This gaseous mixture converts the mercury drop into a gas-mercury foam. In this process the radius of the mercury drop increases, reaching a limiting value under the given conditions, and accumulation of the element being determined proceeds on a surface 10–30 times larger than the initial one. Owing to this, electrolysis is considerably accelerated; during the given electrolysis time, greater accumulation of the element being determined occurs in the mercury drop, and the sensitivity of the determination increases.

In work [2] the possibility was investigated of increasing the sensitivity of determining a number of elements in nonaqueous solutions by simultaneous use of the AAE in combination with increasing the temperature of the solution. In work [3], on the basis of certain simplifying assumptions, a theoretical expression was obtained for the dependence of the radius of a drop of ammonium-amalgam foam on the electrolysis time in aqueous solutions of ammonium salts.

The kinetics of drop growth and the limiting value of its surface depend to a significant degree on the conditions under which electrolysis is carried out. Systematic experimental data on this question are absent in the literature. In connection with this, in the present work a study was carried out to investigate the influence of a number of factors (temperature, electrolysis time, and concentration of the ammonium salt) on the depth of the anodic notch of lead or thallium and on the kinetics of growth and limiting value of the radius (surface) of the ammonium-amalgam foam drop.

The experiments were carried out in an electrolytic cell made of optical glass with plane-parallel walls, which eliminated distortion of the drop during observation and measurement of its dimensions with an MIN-2 microscope (×37; one scale division = 0.05 mm). As the cathode, a mercury drop on a platinum contact was used, obtained by electrolysis of a saturated solution of mercurous nitrate for 1 min at a current of 30 mA. Mercury poured onto the bottom of the cell served as the anode. Exceptions were the experiments studying the influence of temperature, where the anode was

served as the normal calomel electrode. Nitrogen was used for stirring the solution and removing oxygen; the work was carried out on an OH-101 polarograph. The influence of individual factors on the depth of the anodic notch of the element being determined and on the kinetics of drop growth was studied by us, as a rule, in two parallel series of experiments: in the presence of EAA and without it. In the absence of EAA, electrolysis was carried out at a voltage of 1.0 volt, measured between the indicator electrode (mercury drop) and the large surface of mercury at the bottom of the electrolyzer. The choice of voltage was dictated by the requirement of attaining the limiting cathodic current of the elements being determined (in our case Pb and Tl) and, at the same time, excluding discharge of the ammonium ion. In the presence of EAA, electrolysis was carried out at a voltage of 3.0 volts¹, sufficient for discharge of the ammonium ion.

It should be noted that, owing to changes in the surface of the mercury drop under the influence of EAA, the electrolysis current and, consequently, the ohmic voltage drop and the electrolysis potential do not remain strictly constant (the potential of the mercury drop, measured relative to the third auxiliary calomel electrode, varied within several hundredths of a volt). However, this small change in the potential of the mercury electrode (observed only in solutions with an ammonium salt concentration equal to or greater than 1 g-equiv/l) did not affect the basic character of the regularities studied, as shown by control experiments. It was shown experimentally that within the voltage range from 1.0 to 3.0 volts the voltage applied to the cell has no noticeable effect on the depth of the anodic notches of lead and thallium. Thus, for example, the depth of the anodic notch of \(6 \cdot 10^{-7}\) g-ion/l lead in 0.1 \(N\) KCl, obtained at voltages of 1.0, 2.0, and 3.0 volts, proved to be respectively \(4.5 \cdot 10^{-8}\), \(4.0 \cdot 10^{-8}\), and \(5.5 \cdot 10^{-8}\) ampere. This indicates that the increase in the depth of the anodic notches of the elements being determined, obtained at a voltage of 3.0 volts, is caused solely by the presence of EAA.

Fig. 1. Change in the pH of a 1 \(N\) \(\mathrm{NH_4Cl}\) solution during electrolysis in the presence of EAA. Solution volume \(V = 15\) ml; \(t = 18^\circ\mathrm{C}\).

Fig. 2. Dependence of the depth of the anodic notch of lead in 0.1 \(M\) \(\mathrm{NH_4Cl}\) solution (curves 1, 2), of the limiting radius (curve 3), and of the time-averaged radius of ammonium amalgam drops (curve 4) on temperature. Curve 1—\(2 \cdot 10^{-5}\) g-ion/l \(\mathrm{Pb}^{+2}\), without EAA; curve 2—\(2 \cdot 10^{-6}\) g-ion/l \(\mathrm{Pb}^{2+}\), in the presence of EAA. Experimental conditions \(V = 10\) ml; \(\tau = 5\) min; pH = 8.

During electrolysis of ammonium salt solutions, one part of the \(\mathrm{NH_4}\) radicals formed upon reduction of the ammonium ion reacts with water at the electrode surface; another part penetrates into the mercury

with the formation of an unstable amalgam, whose decomposition products are ammonia and hydrogen.

As should be expected, ammonia leaving the drop, interacting with water, alkalizes the solution; therefore the pH of the solution changes, increasing to a certain constant value corresponding to the spontaneously formed buffer mixture $NH_4OH + NH_4Cl$ (or another ammonium salt) (Fig. 1). Therefore, in experiments with EAA, in order to prevent a change in pH before the experiment, 2 drops of $0.1$ $NH_4OH$ (to pH $= 6.5$) were added to the polarographic solution (10 ml). It should be mentioned that in solutions with $\mathrm{pH}<5$ we did not observe the ammonium-amalgam effect, which is in agreement with the literature data [4].

The influence of temperature on the limiting radius of the ammonium-amalgam drop, its time-averaged radius, and the depth of the anodic notch of lead with EAA and without it is presented in Fig. 2. The experiments show that, with an increase in temperature to 45–48°C, the limiting radius of the ammonium-amalgam drop, its time-averaged radius, and the depth of the lead notch increase. A further increase in temperature leads to the formation of a pulsating electrode; at temperatures above 50°C the rates of formation and decomposition of the ammonium amalgam increase to such values at which the drop, as it were, pulsates, alternately increasing and decreasing in volume, and the more frequently the higher the temperature. Thus, the limiting radius of the pulsating electrode (the electrode at temperatures above 50°C) decreases with increasing temperature. As for the time-averaged radius of the ammonium-amalgam drop, it has the greatest values at 50–60°C. The temperature dependence of the depth of the anodic notch of lead with EAA (curve 2) is in agreement with the temperature dependence of the time-averaged radius of the electrode.

Fig. 3. Dependence of the depth of the anodic notch of lead in a 1 M solution of $(NH_4)_2SO_4$ on the electrolysis time with EAA (curve 2) and without EAA (curve 1). Curve 1—$1 \cdot 10^{-5}$ g-ion/l $Pb^{2+}$; curve 2—$3 \cdot 10^{-7}$ g-ion/l $Pb^{2+}$. Experimental conditions: $V = 10$ ml; $\mathrm{pH} = 6.6$; $t = 18^\circ C$.

Fig. 4. Dependence of the radius of the ammonium-amalgam drop on the electrolysis time for solutions of $(NH_4)_2SO_4$ of various concentrations. Curve 1—$C_{(NH_4)_2SO_4} = 0.05$ M; curve 2—$C_{(NH_4)_2SO_4} = 0.1$ M; curve 3—$C_{(NH_4)_2SO_4} = 0.5$ M; curve 4—$C_{(NH_4)_2SO_4} = 1.0$ M.

Fig. 5. Dependence of the depth of the anodic notch of thallium on the concentration of the $(NH_4)_2SO_4$ solution. Experimental conditions: $V = 5$ ml; $\tau = 6$ min; $\mathrm{pH} = 6.5$; $t = 18^\circ C$.

Figure 3 presents the dependence of the depth of the anodic thallium spike on the electrolysis time. It is evident from the figure that this dependence has the form of an exponential that passes into a straight line, and that the greatest increase in the sensitivity of determination due to the AAE is achieved at longer electrolysis times.

A study of the effect of the concentration of ammonium salt on the growth kinetics of the ammonium amalgam drop and on the depth of the anodic thallium spike (Figs. 4, 5) showed that increasing the concentration of ammonium salt in the solution increases only the growth rate of the ammonium amalgam drop, but not its limiting radius.

Conclusions

1. The influence on the ammonium amalgam effect (AAE) of the following factors has been studied: temperature, electrolysis time, and ammonium salt concentration.

2. The conditions for obtaining a pulsating electrode under conditions of ammonium amalgam and elevated temperature have been determined.

3. It has been shown that the dependence of the depth of the anodic thallium spike on the electrolysis time has the form of an exponential that passes into a straight line.

4. It has been shown that increasing the concentration of ammonium salt in the solution does not affect the limiting size of ammonium amalgam drops, but does affect the growth kinetics of the drop. A significant increase in the depth of the thallium spike with increasing ammonium salt concentration cannot be explained solely by a change in the growth kinetics of the drop.

References

1. A. G. Stromberg, Zavod. labor., 10, 1175, 1965.
2. Yu. A. Karbainov, A. G. Stromberg, Zh. analit. khimii, 20, 769, 1965.
3. A. G. Stromberg, A. V. Konkova, Zh. Elektrokhimiya (in press).
4. R. I. Johnston, A. R. Ubbelohde. J. Chem. Soc. (London), 1951, 1731.

1. In experiments studying the influence of temperature, electrolysis was carried out at a voltage of 4.0 volts. ↩